When
I became interested in growing Venus
Fly Traps I reasoned that it would be more convenient to grow
them inside under artificial lights. That way I could put them on an
eye-level rack to make feeding them and watching them easier. The
problem to solve was determining what type of lights to use. This
page chronicles what I discovered from reading dozens of artificial
lighting websites on the Internet and how applying this information
worked for my plants.

There
are two primary factors that must be considered when setting up an
artificial lighting system: amount
of light and quality
of light.

The
amount of light is the product of its intensity (how bright it is)
multiplied by how long it shines on the plant. Outdoors, sunlight
goes through a daily cycle of starting off dim, building toward a
maximum around noon, then weakening until sunset.

Brightness versus time curve for unshaded sunlight on a clear and
bright day

The
area under the bell-shaped curve is the total amount of light the
plant receives from the sun. Because it's difficult to reproduce the
sun's intensity indoors, an artificial system has to be left on
longer to compensate for its reduced brilliance, 12 to 18 hours a day
is usually required. (Except under special conditions and for
particular plants leaving the lights on 24 hours a day is not
recommended. Most plants require some darkness everyday for proper
growth.) Although artificial lighting is usually not as bright as
natural sunlight, it has the advantage of being constant as long as
it's on. Unlike the bell curve for natural sunlight this creates a
constant rectangular area, as shown by the "fluorescent
intensity" area on the graph. As long as the areas under both
curves are close the plants should grow at approximately the same rate.

At
first glance it would appear that even with being on for 18 hours
the fluorescent lamps provide much less total energy (18 units) than
the sun (96 units.) But, for growth plants only absorb 25-percent of
the solar spectrum. All of the rest, mainly in the green and yellow
zone, is wasted. Multiplying the sun's 96 units by 0.25 gives 24
units of energy useable by the plant, still more than the 18 from the
fluorescent lights but close enough for good growth. (Another way of
thinking of this is that the light from fluorescent grow lights looks
four times brighter to plants than they do to us.)

If
you want to measure the intensity of your own lights for comparison
to the sun you can do so with
almost any camera that has a manual mode. Set the aperture to f5.0,
the ISO to 200 and focus the camera on a 17-percent gray card
purchased from any camera store, being careful to avoid angles that
send glare into the lens and so that the card completely fills the
field of view. Adjust the shutter speed to get a good exposure. The
brighter the light the shorter the shutter speed has to be set. This
represents your yardstick: the shorter the shutter duration the
brighter the light.

In
my experiments, using a Canon EOS 20D camera, I found that shade and
overcast days required a shutter speed of 1/1000 second, noon time
sun was around 1/2500, and the light set-up for my plants is only
1/200 second but remember, almost 100-percent of this light is used
by the plants whereas only 25-percent of the sun's light is used.
There are special-made light meters designed to measure the amount of
light utilized by plants. These are not the cheap units available in
hardware stores or nurseries and can be quite expensive.

As
already mentioned in the discussion on intensity, the second primary
factor in artificial lighting is the quality of light. Oddly, with
this factor we don't necessarily want to copy the sun. Consider the
following graph showing the amount of power provided by the sun as a
function of wavelength:

Solar
Spectrum or Spectral Power Distribution Curve

The
sun is brightest at around 520 nanometers, corresponding to a black
body radiating at 5,800 degrees Kelvin. Light at this frequency is
yellow-green and is the color to which our eyes have adapted for
maximum sensitivity. Curiously, this is not the color to which plants
have optimized their absorption to drive their growth. Instead, they
use primarily blue and red light as the following graph shows:

Plant
Absorption Spectrum

The
reason plants look green is that they reflect most of the green
light away (as evidenced by the very low amount of green and yellow
absorbed) while using the blue (for energy, chlorophyll production,
and stem growth) and red (root development, tuber formation,
dormancy, and reproduction.)

This
gives an advantage to artificial lighting because it enables the
spectrum of the light source to be tailored to match the plant's
requirements without wasting a lot of power generating light at
frequencies that the plant can't use. Since the lights are going to
be on a lot of time, a high-efficiency system is desirable to keep
electricity costs down.

Besides
the primary factors of light quantity and quality, anyone wishing to
use artificial lights for growing plants needs to consider secondary
factors such as purchase cost, ease of installation, maintenance,
electricity costs, heating problems caused by the lights, and so on.

After
evaluating all these factors I decided the optimal solution for my
lighting needs was a fluorescent system using two, 48-inch 2-tube
fixtures. The fixtures were on sale for only $8.00 each at Walmart,
installed easily, the tubes last for years (more on that later) are
electrically the most efficient producers of light, they produce very
little heat, and the spectrum (through a judicious selection of tube
type) is the closest match to the plant's needs.

Many
websites recommend high intensity discharge systems because they
provide the brightest light and can produce a spectrum very close to
sunlight. The problem with such systems for me is that they produce
so much heat that they would create cooling problems in my desert
location. Also, a small system like I need would have been several
times as expensive and complicated to install as a fluorescent one
and would be extremely costly to operate continuously.

So,
fluorescents were the best option for me. The problem is that
although they look bright inside they're are very weak compared to
sunlight. Leaving them on for a large part of the day helps
compensate for this as well as enclosing the growing chamber with
walls painted white to reflect as much light as possible back onto
the plants rather than escaping out of the sides and being wasted.

The
light from a point source like a normal incandescent bulb falls off
by the square of the distance: doubling the distance from the bulb
reduces the intensity by a factor of four. Because a fluorescent tube
is a long source rather than a point source, intensity falls off more
linearly when the plant is close: doubling the distance reduces the
intensity by half. With a 2-bulb fixture in a reflector (the typical
and inexpensive shop light set-up) the intensity falls off even
slower, about 1.5 times the increases in distance. For my setup, with
the lights and plants enclosed in a white box, the lighting is almost
perfectly even so that it's constant within 8 inches of the bulbs.
This is convenient because it means I don't need to continually
adjust the elevation of the plants as they grow. Better still, all
that light being reflected back onto the plants increases the
intensity of the light by a factor of two. Just as important is that
the cool running of the fluorescent bulbs means that the inside of
the growing chamber doesn't heat up as it would with a HID system.

The
last issue to be resolved was what type of bulbs to install in the
fixtures. My first thought was to use bulbs labeled as
"daylight," thinking that they would be the most natural.
The problem, as the following spectral power distribution curve
indicates, is that while these produce enough blue light they are
weak in the red zone and waste a lot of electricity producing lots of
green and yellow light.

Daylight
Deluxe Light (6500 K)

I
read in several sites that using a combination of warm spectrum
lights (for the red) and daylight or cool lights (for the blue) was a
good way to go. The following spectrum of a warm light suggests that
this might work, it's certainly rich in red. (Note: non-deluxe or
non-plus type daylight bulbs are weak in the red zone. I also found
that cool-white types are weaker in blues and greens than daylight types.)

Warm
Light Deluxe Light (4100 K)

The problem is
that such mixtures of bulbs still produce an over abundance of green
and yellow light, which is a waste of energy. (Note: non-deluxe or
non-plus type lights are also weak in the red zone.)

Fortunately,
the fluorescent light industry has come to the rescue by producing
special plant growth lights with phosphors selected to produce light
in a spectrum that almost perfectly matches the absorption spectrum
of most plants, as the following graph shows:

Plant/Aquarium
Light

It
is important to note that the vertical scale on this last graph is
much more compressed than on the previous two. Plotted to their
scales this chart would have power maximums that were three times
higher than the others. I assume from this that plant lights appear
to be very efficient in producing only the right type of light. The
lights I use are GE Plant and Aquarium Wide Spectrum lights. (I've
read that Philips produces a grow light called Agro-Lite that is
supposed to make plants grow 2-10 percent faster, but none of the
stores in my area carry it.)

The
sharp spikes in the spectral power distribution curves are the
colors emitted by the mercury vapor inside the tubes as electrons
flow through it. The more continuous spectrum, and the source of most
of the bulb's light, is from ultraviolet light from the mercury
striking the phosphor coating inside the tubes and making it glow.
One source claimed that while fluorescent bulbs may last many years,
the light output for growing plants decreases significantly with time
and should therefore be replaced every 6 months. The source failed to
mention if this was 6 months of actual "on" time or just
daily use.

My
fluorescent light growing chamber, small but efficient.

How
does it work? Unfortunately Venus Fly Traps grow so slowly that it's
hard to make comparisons. I have been able to do so by growing
cantaloupes for transplanting into my garden. In 8 weeks I grew a
plant from seed to one with several 18-inch long vines. This required
leaving the lights on 18 hours a day. The same type of melon grown
outside in sunlight grew 50 percent larger in the same amount of
time. Both plants were equally heathy and robust.

Beware
of "Watts-Per-Square-Foot"
Recommendations

I
came across many of these while researching my system, ranged from
10 to 40 watts per square foot. The problem with such recommendations
is that they say practically nothing about the amount of light placed
on a plant. First, different type bulbs vary greatly in the amount of
total light (measured in lumens) they produce. Incandescent bulbs can
produce as little as 13 lumens per watt whereas fluorescents can be
as high as 85 lumens per watt. Second, bulb shape, reflector
configuration and the distance of the plant from the light have an
enormous impact on the amount of light a plant receives. Finally, the
color or spectrum of the light greatly influences how well the plant
can use the light hitting it. Only 20-percent of the light from an
incandescent lamp is used while virtually 100-percent of the light
from a good fluorescent grow lamp will be absorbed.

Consider
a 100-watt bare incandescent bulb 1 foot away from a plant covering
one square foot of area. Such a lighting set-up will shine 108 lumens
onto the plant's one square foot of which the plant will only be able
to use 20 lumens. This is so dim the plant might as well be locked in
a dark closet. One the other hand, consider a 100 watt fluorescent
grow light equipped with a good reflector. It will produce 8,000
lumens, of which 7200 can be channeled onto and used by the plant
(some losses are unavoidable.) This is enough light for any plant to
flourish. Clearly, citing a "watts-per-square-foot"
recommendation is not the best way to design a plant lighting system.

Another
problem to consider is the design of the bulb's reflector. Many long
fluorescent fixture place the bulb so close to the reflectors that
much of the light is trapped and lost behind the bulbs.

Beware
of blue "plant lights"

I've
seen spotlights with the front painted blue being sold as "plant
lights." When I read the description, it always states that
they "Make plants look greener." These colored
lights just make the plant look good, they aren't the same as grow lights.

How
does my
inside system compare to growing plants outside in sunlight?

By
measuring the rate of leaf growth I was able to estimate that my
lighting system grows plants about as fast as plants placed outside
against an east-facing wall where they only get direct morning sun
until 1 PM. Plants growing against a south-facing wall painted white
to reflect extra light into them grow twice as fast as those under my system.

IMPORTANT
UPDATE!!!

Plant
Growth Comparison

Several
months after posting this page I started doing Internet searches for
information on which type of fluorescent grow lamp produced the best
growth. I was surprised to discover than I couldn't find a single,
unbiased comparison test so I desided to conduct my own.

I
found four different fluorescent plant lights at the local hardware
stores: GE Plant and Aquarium, Sylvania Gro-Lux wide spectrum,
Philips Plant and Aquarium and the Ott-Lite Plant
Growth
light. As a standard, I purchased GE Kitchen and Bath bulbs
because they had the brighest lumen rating. Here's what they look like:

I
constructed five identical growing boxes, one for each lamp type and
a fifth for the brightest fluorescents I could find: GE's Kitchen and
Bath bulbs which put out 3400 lumens each compared to the 1600-1800
for the plant and aquarium bulbs. I planted the same number and type
of Zinnias, tomatoes, Salvias and melon seeds in each growing box.
All the boxes were kept in the same room and positioned to ensure
they all remained at the same temperature and had the same access to
fresh air. I was careful to make certain that they all used the same
potting soil and received the same amount of water.

Additionally,
I set up an identical set of planters for outside growth. These will
show how well the artificial lighting systems work compared to
nautural sunlight. These were brought in every night and kept in the
same room as the artificially lit plants to maintain as many
variables the same as possible.

Results:

The
following image shows how a single variety of zinnia grew under the
various lighting systems:

GE
Plant and Aquarium Wide Spectrum lights clearly did the best with GE
Kitchen and Bath lights a close second. The much-touted Ott Lites
were the worst. Most remarkable is that many plants did better under
the artificial conditions than those grown in natural sunlight. I
assume this is because the artificially lit plants had more uniform
growing conditions. I got essentially the same results using tomatoes
and melons as the test plants. What this experiment convinced me of
is that in the future I will be using GE's Plant and Aquarium Wide
Spectrum lights for all my artificial plant lighting.

My
first system making use of all this information consisted of a
15-inch wide by 48-inch long bright white box with two light fixtures
loaded with four GE Plant and aquarium bulbs. A small fan at one end
blows through the box to provide a constant supply of fresh air for
them to breath as well as to prevent the build up of excess humidity,
which can lead to diseases such as damping off. Each side consists of
two sheets of perforated masonite thumb-screwed together. As the
plants get taller one of the two sheets on each side can be lowered
to give the plants more head room. Although this box worked well, it
was too small (only holds 30, one-quart planters) and moving the
sides up and down got to be a hassle.

My
second system was 2-feet wide and high and again 4-feet long,
providing enough room for 50 quart-sized planters. By painting the
top, sides, ends and bottom bright white enough light is retained so
that for all intents and purposes the light strength is constant over
the entire height of the box. This means the plants don't have to be
moved up or down during the growing season. Three, 2-bulb fixtures
screwed to the underside of the top provide enough light so seedlings
grow fast, dark green and stocky, indicating that they are receiving
plenty of light.

The
one thing that I still didn't like about such growing boxes is that
the reflectors on the light fixtures are so close to the tops of the
bulbs that much of the light given off is trapped between the bulb
and reflector and therefore wasted.

To
address this problem I built a second version of the larger box,
this time with the bulbs mounted directly to the box. A series of
"V" shaped reflectors were positioned over the tubes so
that the maximum amount of light was extracted from the back side of
the bulb and redirected down toward the plants.

Comparing
the two systems with a light meter showed that the improved design
increased the amount of light on the plants by 25-percent. As good as
this sounds I doubt I will modify the first larger box because the
amount of work required is enormous and extremely tedious. Besides,
the first big box does just fine.

T5
Bulbs :

I've
been following the growth in popularity of the new T5 fluorescent
bulb systems and have to admit that at first blush they sound pretty
good. T5 bulbs are smaller in diameter than standard fluorescent
bulbs, making reflector design easier and more efficient. They
produce more light per watt than regular fluorescents, sometimes as
high as 95 lumens per watt.

However,
to my thinking there are two problems with the new bulbs. First, the
bulbs, holders and reflectors cost ten times as much as standard
systems yet only produce 25-percent more light. Second, although they
are specifically targeted at the indoor gardener there is very little
information available about spectrum optimization. For example, many
of these "plant" bulbs are full spectrum lights that mimic
the solar spectrum. As already discussed, this is not efficient for
artificial lighting schemes. Also, as of May of 2009 no one I've
found makes a true "plant light," one that have its maximum
intensity in the blue and red zones. Rather they only offer dedicated
blue and red bulbs that have to be mixed and matched to produce an
optimum mix spectrum. This might be acceptable but the spectral
distribution of these lights isn't provided. How sharp are the peaks?
How many sharp spikes are there and how does their intensity relate
to the power in the phosphor light profile? How many red lights
should be mixed with blue bulbs for optimum growing conditions?

With
so many unanswered questions coupled with the exorbitant price this
is a technology I can do without until it matures. For $15 I can add
an extra set of lights to one of my grow boxes and produce just as
many lumens as a high-tech T5 system costing over $200.

Fluorescent
Bulb Brightness Test:

I've
read that fluorescent bulbs weaken with time so that after six
months their light output is cut in half. The problem with such
claims is that I haven't found one that defines the duty cycle:
whether they were used 10 hours a day, 14, or 18. To answer this
question in March of 2009 I set up a GE Plant and Aquarium bulb on a
timer to cycle it on for 16 hours a day. In front of it I secured a
digital camera to record the amount of light given off. The camera's
settings were selected so that a decrease of less than 10-percent
would be detected. Here's what I found:

April
(31 total on-off cycles): no decrease in brightness.May
(61 total on-off cycles): no decrease in brightness.June
(92 total on-off cycles): no decrease in brightness.July
(123 total on-off cycles): no decrease in brightness.Aug
(155 total on-off cycles): a 10-percent decrease compared to the
original brightness.Sep
(186 total on-off cycles): a 10-percent decrease compared to the
original brightness (in other words: no change since last month.)

At
this point I'm considering this experiment over. It established that
at the settings used, GE Plant and Aquarium wide spectrum fluorescent
bulbs do not loose half their brightness in six months. They do get
weaker, but only by 10-percent.

So
how long is a fluorescent bulb good? In as much as the amount of
light in an enclosed light box, such as those described on this page,
is borderline for many plants, I intend replacing my bulbs after six
months of use.

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